MPlayer

Mplayer is a powerful media player designed for Linux and other UNIX like systems. MPlayer allows playing almost all formats, including Windows media files and QuickTime files using both: native codecs and Win32 codecs. The mplayer-plugin allows users to play streaming media inside mozilla based browsers, such as mozilla, firefox and konqueror.

Even though this player was originally created for Linux and UNIX there is an unstable beta release for Windows.

As mplayer is licenced under GPL anyone can download and modify the source code for mplayer which means that mplayer always changes and new releases are available frequently for download. There is also third party software that uses mplayer, such as the plugger plugin for mozilla browsers, which allows playing almost all media files, displaying word documents, photoshop images and much more.